Existing approaches for reading RFID labels employ a traditional antenna that provides the large read range for RFID labels. This approach provides a majority of the antenna energy to be used in the far field. The far field region is defined as distance
      d    >>          λ              2        ⁢                                  ⁢        π              ,where λ is the wavelength. For the UHF frequency 915 MHz, this value is about 5 cm. So, the far field region at 915 MHz is substantially beyond 5 cm, and similarly the near field region is substantially below 5 cm. Most RFID reader antennas are designed to read labels at the highest distances of several meters for example, which of course is well in the far field region.
In certain applications, namely RFID label applicators and programmers, it is desirable to read and write only one RFID label within a group of labels located in close proximity to each other. For example, on a label applicator machine, labels are packaged on a reel to facilitate processing on the machine. On the reel, the labels are placed side-by-side or end-to-end in close proximity. However, it is difficult for a traditional UHF antenna to direct energy to only one label at a time, due to the fact that the traditional UHF antenna generally has a broad radiation pattern and directs energy well into the far field. The broad radiation pattern illuminates all RFID labels within the range of the antenna. If an attempt is made to write the product code or serial number to one label, all illuminated labels are programmed with the same code or serial number.
A traditional far-field radiating antenna used in such RFID UHF applications is a patch antenna. Usually the patch area which radiates is fed through a connector energized by RFID electronics. Typically a conducting plate is mounted on the backside and spaced a small distance from the patch area.
For those applications mentioned above where it is desirable to read or write information to an RFID label at very close distances, such as label applicators where one label at a time needs to be programmed, tested, and applied, traditional far field antennas perform poorly. Traditional radiating antennas require that tagged items be separated by substantial distances in order to prevent multiple items from being read or programmed simultaneously or require usage of metal windows to shield all labels except the label being programmed or read.
However, such techniques do not adequately solve the problem because if the labels are spaced further apart, the applicator throughput is lowered and the number of labels in a given reel size is limited. If shield techniques are used, a different shield is required for each different label shape and spacing. Therefore, changes are required to process different labels on an applicator line, also effectively lowering throughput.